that presents major challenge to law enforcement. abc's david kerley has more on this. >> reporter: the bloody boston explosion, shattered the marathon and reopened wounds in this country. this attack far from the first targeting civilians. international terrorists tried to bring down the world trade center in '93. the building withstood the blast, killing six. and domestic terrorists have proved deadly as well. the oklahoma city bombing in '95 killing 168. so similar to the boston blast, the atlanta olympics bombing in '96. spectators gathered in a park when the bomb went off. it is the soft target terrorists are looking for. >> unfortunately, these kind of settings, whether it's a marathon, football, baseball, social event, concert, are attractive targets. we just can't get around it. >> reporter: tom ridge was the country's first secretary of homeland security. >> you have to manage the risk. unless you have specific information about a particular actor, you can't guarantee safety anywhere. >> reporter: so even though police were